I posted a very lengthy comparison of the E4 and the FS2000 over at arfcom but have since been banned, let me see if I can track it down.
In summary, I have shot both extensively and prefer the weight and price of the E4, but prefer the looks, ergos, and reliability of the FS2000.
The FS is more expensive, but not near at finicky as the E4 I shot. MSAR makes a great weapon, but the one that I personally shot was less than reliable.
ETA: Basically these were my findings-
The AUG Clone- If you want this gun, get the E4, not the STG model. The STG model uses mags that seem to be less reliable, whereas the E4 uses AR mags (PMags included). The gun is lighter, slimmer, and cheaper than the FS2000. It is slightly less complex internally, since it uses a more conventional ejection mechanism. However, the one I shot was inconsistent in its reliability. I shot about 350 rounds and it failed to eject about five times. Random ammo types, different mags, so it was just problematic, no direct cause. Many users on the other hand report flawless reliability after a "break in period". No offense to MSAR, but FN definately has a solid lineage of building quality weapons. MSAR is a newcomer.
The FS2000-Bulkier, more expensive, heavier than the AUG clone. Only uses metal AR mags, no polymers. Get the CProducts mags and youre fine. Fully ambidextrous without moving around of internals. More reliable with all types of ammo (no jams for me in a couple thousand rds) even with steel cased. I think the FS2000 looks better, but thats personal preference. The biggest downside to the FS is the inability to effectively mount a light on it without having to buy the expensive (160) trirail unit. However, without sounding like an ass, neither of these are budget weapons, so some expense comes with the territory.
My findings: both guns are sexy looking head turners at the range, but Id trust my life to the FS first. You can find a used one for not much more than the E4 sells for.